The goal of our studies is to understand the molecular mechanisms of paramyxovirus membrane fusion using Newcastle disease virus as a prototype. As in most paramyxovirus systems, membrane fusion requires both HN and F proteins. Our studies have focused on how F protein mediates membrane fusion and the role of HN protein in that process. We have recently made four observations that have a direct bearing on unresolved questions about paramyxovirus fusion. First, we have found that a specific domain of the F protein interacts with a domains of the HN protein providing direct evidence for an interaction between sequences from the two proteins and identifying specific domains of the F protein involved. Second, using antisera specific for F protein HR1 and HR2 domains, we have demonstrated conformational differences in F protein dependent upon HN protein expression and F protein cleavage. Third, using peptide antisera specific for a short sequence of the HN protein, we have demonstrated conformational differences in HN protein related to F protein expression and cleavage. Fourth, we have made a point mutant in the F protein that eliminates the absolute requirement for HN protein in syncytia formation, a finding that raises questions about the role of HN protein attachment in fusion. To address the questions raised by these findings we propose four specific aims: 1) characterize interactions between specific domains of the HN protein and F protein, 2) characterize conformational changes in the F protein related to HN protein expression and F protein cleavage, 3) characterize conformational changes in the HN protein related to HN protein attachment and F protein expression and cleavage. 4) explore the role of attachment in initiation of fusion.